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WA435 - Relation Between the Dungeness River and the Shallow Aquifer in the Sequim-Dungeness Area, Clallam County, Washington - Completed FY2002
Problem - The Sequim-Dungeness area is about 65 square miles and is located in northwestern Washington State (fig. 1). The Dungeness River and its tributaries drain about 200 square miles, mostly in the Olympic Mountains to the south. After emerging from the mountains, the river flows for about 11 miles northward across the study area before emptying into the Strait of Juan de Fuca. A shallow water-table aquifer underlies the study area. It is mostly coarse deposits of alluvium and glacial outwash and its thickness averages about 100 feet. The Dungeness River and the shallow aquifer are closely related. Some reaches of the river recharge ground water and some reaches of the river receive ground-water discharge. However, this is a complex relation and the magnitudes and seasonal variations of ground-water recharge and discharge are uncertain or unknown. A better understanding of the relation between the Dungeness River and the shallow aquifer is needed to effectively manage the water resources of the study area. Competing needs are drinking water from ground water, irrigation water from the Dungeness River, and salmon habitat in the Dungeness River.
Objectives - The broad objective of this proposed study is to describe the relation between the Dungeness River and the shallow aquifer. The study will describe the annual and seasonal longitudinal distributions of flow quantities and water-level gradients between the river and the aquifer.
Relevance and Benefits - This study will improve the understanding of the relation between the Dungeness River and the shallow aquifer in the Sequim-Dungeness area of northwest Washington. This improved understanding can benefit other areas of the Pacific Northwest, because the hydrology of the Sequim-Dungeness area and the emerging social, economic, and biologic issues are similar to other areas of the Pacific Northwest. Ground-water discharge provides an important source of streamflow especially during the low-flow periods of summer and fall. In some areas, leakage from rivers provides an important source of ground-water recharge. The demand for ground water is increasing in many areas because of a rapidly growing population. At the same time, most salmon populations in Pacific Northwest rivers have been declining and the Chinook Salmon was recently placed on threatened status as part of the Endangered Species Act. Minimum levels of streamflow are needed to sustain and improve salmon populations, and there is concern that increased ground-water withdrawals will decrease streamflows below the minimum requirements.
Local, State, and Federal agencies in the Pacific Northwest need to work together to balance the demands for water from a growing population with the requirements of minimum streamflow levels for salmon. Knowledge about the relation between rivers and ground water is crucial to make informed decisions about granting new water rights, planning economic development, and managing natural resources. Methods or tools are needed to determine the relation between rivers and ground water, and this study will gain information about the value of a method that can be applied to other areas of the Pacific Northwest. In addition to the specific information for the Dungeness River, the concepts and general relations learned in this study can be applied to other areas of the Pacific Northwest.
This study is relevant and provides benefits to the national USGS goals and mission and to important issues identified in the USGS Washington Water Science Center 1999 Science Plan. The study addresses one of the nine priority issues outlined in the Strategic Directions of the Water Resources Division, 1999-2008. Issue number (8) is surface-water and ground-water interactions as related to water-resource management. This study will provide information that can be used to evaluate the effects of ground-water withdrawals on streamflows and it will estimate the longitudinal and seasonal distribution of the flows between the Dungeness River and ground water.
This study is appropriate for the role of the USGS in the Federal-State Cooperative Program because it will provide information that fits into two of the broad goals specified in WRD Memorandum 95.44. Information for goal number (2) (advancing field or analytical methodology) will be the knowledge gained on how useful it is to apply analytical methods to water-level data on ground-water/surface-water interactions. Information for goal number (3) (advancing understanding of hydrologic processes) will be the detailed understanding of the longitudinal and seasonal distribution of flows between the Dungeness River and ground water.
The USGS Washington Water Science Center 1999 Science Plan identified five important water-resource issues in the State of Washington. The District is focusing its efforts on advancing hydrologic methods related to these issues and on improving the understanding of the hydrology affecting these issues. This study will provide information on one of the five issues-- water availability. This study specifically addresses one of the subissues for water availability -- ground-water/surface-water interactions and declining ground-water levels.
1) The Dungeness River will be divided into six reaches and two seepage runs will be conducted, once in the fall and once in the spring.
2a) One gaining and one losing reach will be selected. In each reach, a surface-water gage will be installed in the river and a transect of six piezometers will be installed perpendicular to the river. Continuous recorders will be installed and water levels will be measured for 15 months. Monthly measurements will be made for water levels, specific conductance, and temperature for 15 months.
2b) A pair of piezometers and a staff gage will be installed in each of the four other seepage-run reaches and monthly measurements will be made for water levels, specific conductance, and temperature for 12 months.
3) The water-level data will be analyzed using analytical solutions for the interaction of a water-table aquifer and stream water levels.
WA393 - Hydrogeologic Assessment of the Dungeness River Basin - Completed FY1999
Problem - The proposed study area comprises about 120 square miles in eastern Clallam County, an area covered largely by unconsolidated glacial drift. The glacial drift in the study area ranges up to about 2,400 feet in thickness and contains most of the water-yielding materials in the study area. Three aquifer units, two semi-confining units, and 1000 feet or more of undifferentiated unconsolidated deposits have been identified in previous studies. As of 1960, the Sequim-Dungeness area was largely agricultural and had a population of about 5000 people. In the mid-1960's, land use in the area began to shift to residential, resulting in populations of about 18,000 in 1992. Domestic water needs of the area are primarily met by ground water from both public-supply and domestic wells. Except for the City of Sequim, which has a sewer system and treatment plant that discharges to marine waters north of Sequim Bay, waste disposal is by individual septic systems and one community septic system that serves 800 residents. A mass sampling effort by the Clallam County Department of Community Development in 1992 revealed an overall increase in nitrate concentrations since 1980. It is not known if these increases are due mostly to (1) increasing numbers of septic systems, (2) changes in agricultural practices, or (3) infiltration of leachates into the aquifers from historical agricultural practices. The shift in land use from irrigated agriculture to residential may have reduced recharge quantities to the aquifer system, which would also tend to increase nitrate concentrations. There is need to update and expand the existing knowledge of the aquifer system, especially with respect to the geohydrologic framework of the area so that the origin and fate of contaminants can be better understood.
Objectives - We will describe and quantify the geohydrologic framework, water levels, and hydraulic properties of the ground-water system to the extent that improved ground-water flow modeling could be undertaken without additional data collection beyond that proposed herein. We will investigate the distribution and probable major sources of nitrate in the ground-water system, and improve our estimates of spatial distribution and rates of recharge to the ground-water system. We will determine the current stresses for each of the known major aquifers for comparison with ground-water recharge estimates for use in possible subsequent modeling of the ground-water system. We will construct a water budget that will include ground-water recharge, pumpage, streamflow, subsurface outflow to marine waters, and return flows from on-site sewage disposal.
Approach - The major project work elements include (1) making a comprehensive inventory of 200 to 300 existing wells in the project area, (2) using measured water levels to draw water-level maps for each aquifer unit where data are sufficient, (3) collecting water-quality samples at 50 to 100 wells to identify sources and areal distribution of nitrate, (4) estimating nitrate loading to the ground-water system, (5) calculating the current distribution of recharge to the ground-water system, (6) collecting discharge data for streams, and (7) determining current stresses on the ground-water system.
WA390 - Aquifer Delineation in the Dungeness River Basin, Clallam County, Washington - Completed FY1995
Problem - A ground-water investigation of the Dungeness River basin in the late 1970's resulted in a limited amount of information regarding the ground-water aquifers of the study area. In particular, only one hydrogeologic section, oriented north-south, depicted the number and stratigraphic locations of aquifers being tapped by wells at that time. Subsequently, a great many wells have been drilled in geographic areas that were largely undeveloped at the time of the earlier study. In addition, several wells have been drilled to greater depths than was common in the late 1970's in an attempt to find new sources of groundwater. County health and planning officials now would like a better understanding of the number, thickness, and geographic extent of aquifers in the lower Dungeness River Basin.
Objectives - The principal objective of this study is to modify, refine, and extend knowledge of the thickness and lithologic characteristics of ground-water aquifers currently being utilized in the lower Dungeness River basin, using only available data.
Approach - The U.S. Geological Survey (USGS) will provide Clallam County, the cooperator, with a computerized list of all field-checked wells in the study area and a plot of those wells, with depth labels, on a map or overlay of scale 1:100,000. In turn, the cooperator will review the list and send USGS a second list of all wells in the Clallam County data base, both checked and unchecked, that are not included on the USGS list. The county list will be accompanied by corresponding drillers' reports for the wells under consideration. The USGS will plot those wells on a second map or overlay, using only wells which have adequate drillers' logs which are either in previous undeveloped areas or are significantly deeper than those available for the late 1970's studys.